There are several potential problems. I notice in your
screenshot the error "Beta expired." If you are running Flex
Builder 3 beta 1, you may be past your 30-day trial. If you are
running beta 2, did you uninstall beta 1 first?
Second, try fully cleaning all of your projects and
rebuilding them. Select Project > Clean > Clean all projects,
and make sure "Start a build immediately" is checked.
Third, you may want to create a fresh workspace. Select File
> Switch Workspace and select an empty folder. Then use File
> Import > Existing Projects to copy your old projects into
the new workspace. Make sure you never share the same workspace
between multiple copies of Flex Builder.
If none of this helps, please let us know by filing a bug at
Attach your Eclipse error log, found at <your
workspace>/.metadata/.log, and include as many other details
about your setup as possible.